For separation of a multiphase flow traditional separation, devices in the form of containers are applied. Such containers are often large and heavy and must be constructed in accordance to design codes for pressure containers. If there is danger of liquid slugs, the container volume must be sufficiently large in order to handle a liquid slug. For application at deep sea depths, conventional separators may be too heavy to be handled with the aid of conventional vessels and lifting devices. If equipments inside a conventional separator are destroyed or lose their effectiveness in time, the whole separator must be raised for replacements, alternatively that the separator must be opened for repair. The separation efficiency in relation to the weight of equipment may advantageously be improved. There is a need for an assembly for multiphase separation with advantageous properties as regards to production, installation, operation, maintenance, replacement of single components in the assembly and separation effect in relation to weight.